When variants of SARS-CoV-2 (the virus that causes COVID-19) emerged in late 2020, concern arose that they might elude protective immune responses generated by prior infection or vaccination, potentially making re-infection more likely or vaccination less effective. To investigate this possibility, researchers from the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, and colleagues analyzed blood cell samples from 30 people who had contracted and recovered from COVID-19 prior to the emergence of virus variants. They found that one key player in the immune response to SARS-CoV-2—the CD8+ T cell—remained active against the virus.